Frozen dataclasses with slots raise TypeError

[treyhunner]

BPO	45897
Nosy	@ericvsmith, @treyhunner, @AlexWaygood
PRs	bpo-45897: Fix frozen-slotted dataclass bug #29895

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = None
created_at = <Date 2021-11-25.01:53:51.202>
labels = ['type-bug', 'library', '3.10', '3.11']
title = 'Frozen dataclasses with slots raise TypeError'
updated_at = <Date 2021-12-02.17:20:40.230>
user = 'https://github.com/treyhunner'

bugs.python.org fields:

activity = <Date 2021-12-02.17:20:40.230>
actor = 'AlexWaygood'
assignee = 'eric.smith'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-11-25.01:53:51.202>
creator = 'trey'
dependencies = []
files = []
hgrepos = []
issue_num = 45897
keywords = ['patch']
message_count = 4.0
messages = ['406973', '406974', '406995', '407540']
nosy_count = 3.0
nosy_names = ['eric.smith', 'trey', 'AlexWaygood']
pr_nums = ['29895']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45897'
versions = ['Python 3.10', 'Python 3.11']

[treyhunner]

When making a dataclass with slots=True and frozen=True, assigning to an invalid attribute raises a TypeError rather than a FrozenInstanceError:

>>> from dataclasses import dataclass
>>> @dataclass(frozen=True, slots=True)
... class Vector:
...     x: float
...     y: float
...     z: float
...
>>> v = Vector(1, 2, 3)
>>> v.a = 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 5, in __setattr__
TypeError: super(type, obj): obj must be an instance or subtype of type

[AlexWaygood]

This looks to be due to the fact that slots=True leads to the creation of an entirely new class (see line 1102), meaning that in the super(cls, self) calls in lines 611 and 618 (in the _frozen_get_del_attr function, responsible for generating __setattr__ and __delattr__ methods), self is no longer an instance of cls.

I believe this can be fixed by tweaking _frozen_get_del_attr so that cls in the generated __setattr__ and __delattr__ methods is dynamically computed (cls = type(self)), rather than read from a closure, as is currently the case.

[ericvsmith]

I think the error should be AttributeError, which is what you'd get if the class weren't frozen.

[AlexWaygood]

You get the same error if you subclass a frozen dataclass, then try to set an attribute that is not one of the superclass's __slots__:

>>> @dataclass(slots=True, frozen=True)
... class Point:
...     x: int
...     y: int
... 
...     
>>> class Subclass(Point): pass
... 
>>> s = Subclass(1, 2)
>>> s.z = 5
Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    s.z = 5
  File "<string>", line 7, in __setattr__
TypeError: super(type, obj): obj must be an instance or subtype of type